TIME-DEPENDENT STRENGTH MOBILIZATION OF CLAY SEAMS EMBEDDED IN PROGRESSIVE ROCK SLOPE FAILURE PLANES

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Many of the rock slope failures take place as the rock mass slide along the slip planes in which thin clay seams are embedded. It is not usually possible to carry the triaxial tests on the undisturbed thin clay seams taken from the slip planes because the thickness of the specimens is not large enough to set on the pedestal of the triaxial apparatus. The only possible way of carrying shear tests on the thin clay specimens seems to use the shear box test which traditionally considered as a minor testing method due to its simple mechanism of shear. The investigators have carried many series of shear box tests on the clay seam specimens taken from the actual rock slope failures and found that the constant volume shear box tests can give reasonable strength parameters in terms of effective stress. They have theoretically derived the method to convert the strength obtained from shear box tests into the strength obtained from triaxial tests. This was successfully utilized in the analyses of … More rock slope failures and resulted in the findings of mechanism of rock slope failures typically triggered by heavy intensive rainfall. The strength parameters to be applied to the slope failure analyses, however, were found from the back-analysis of case studies to be of zero cohesion even in the cases that some cohesion in terms of effective stress was actually observed ion the shear box tests. This zero-cohesion was eventually found to be resulted from the submersion of the clay seams undergoing the progressive rock slope failure during the period of occasional heavy rainfalls. The conclusion most important from the practical point of view is that the rainfall was found to be effective not only in inducing the driving force of slope failure by high water pressure in the slip planes but also in temporarily reducing the cohesion of the clay seams caused by submersion. This implies that we should not rely on the cohesion of the clay seams in estimating the possible factor of safety of rock slope failures. Less